Experimental investigation of the cementation and tensile properties of cement to formation interface considering surface irregularity and drilling mud residue

Cement sheath is an annular structure situated between casing and formation, which is designed to seal the formation and reinforce the borehole. However, owing to the surface irregularity and drilling mud residue, the interface between the cement sheath and the formation is one of the weakest parts for fluid leakage causing well integrity issues, which would also affect the reliability of the cement bond logs. To evaluate the cementation state and tensile properties of the cement-formation interface, rock-cement composite disc specimens are prepared, considering key factors such as lithology, interface irregularity, treatment of oil-based drilling mud contamination, and flushing fluid washing. Flushing efficiency of the drilling mud contaminated rock surface, interface cementation characteristic, referenced tensile strength, and tensile fracture morphology are acquired and analysed comparatively. Results show that (1) the flushing efficiencies of shale and sandstone are 74.1 ~ 61.9% and 52.4 ~ 28.4%, respectively, decreasing with the increase in surface irregularity. (2) The rock-cement interface gradually becomes poorly cemented as the surface cleanliness decreasing and the irregularity increasing. (3) Under the condition of drilling fluid contamination, there exists a transitional area at the cemented interface, where microcracks and pores are developed. (4) With the influence of drilling mud contamination, the referenced tensile strengths of shale-cement and sandstone-cement interfaces drop from 2.716 to 0.586 MPa and from 2.840 to 0.007 MPa, respectively. The effects of underground temperature and pressure environment are not particularly considered in this study and would be investigated systematically in future research.